1. Field of the Invention
This invention relates to apparatus for optically reconstructing information, and more particularly to apparatus for optically reading out information recorded on a disc surface.
2. Description of the Prior Art
Apparatus for optically reconstructing recorded information have heretofore been provided in the form of an optical video disc, an optical information retrieval system, an optical record, etc. For example, in the case of the optical video disc, dot information recorded concentrically or spirally on a disc surface rotates with the rotation of the disc. In order to read out the information, a laser beam produced by a laser source illuminates the rotating disc, and light reflected from the information or transmitted therethrough is received by a photodetector. The information is read in the sequence of the recording. A picture or a sound is reconstructed from the information read out.
However, this type of optical read-out apparatus requires a high precision in tracking the information. Hereunder, the extent of tracking precision which is needed will be explained by taking the optical video disc as an example.
The pitch of the arrayal of the information disposed on the disc is 1.5 - 2.0.mu.m, while the size of each dot of the information is approximately 0.6 - 1.3 .mu.m. According to the manufacturing standards of the video disc, etc., the allowance of the machine precision of a hole at the center of the disc is 100 .mu.m. The disc can therefore have an eccentricity of 100 .mu.m. On the other hand, it is subjected to a high speed rotation of, for example, 1,800 revolutions/min. Under such severe conditions, the required tracking precision must be about 1.5 -2.0.mu.m in the pitch direction, and about 1.0 .mu.m in the direction perpendicular to the pitch direction.
For this reason, the desired tracking precision is usually achieved by utilizing a feedback control system, as will be described below. As seen in FIG. 1A, a laser beam 2 emergent from a laser 1 is directed onto a movable mirror 3, and a reflected light beam 2' from the mirror is guided through a lens 4 onto one surface 5' of a disc 5 so as to illuminate a dot-shaped information 6 (shown in FIG. 1B) recorded on the disc surface 5'. Simultaneously therewith, in order that the reflected light beam 2' may track the information, an output 9 from a detector 8 for reading the information controls the deflection mirror 3 through a servo circuit 10.
In order to form the laser beam 2, 2' into a spot having a size of 1 - 2.0 .mu.m on the disc surface, the focal plane of the lens 4 should exactly coincide with the disc surface 5'. To this end, the lens 4 is supported by a voice coil 11 and is controlled through a servo circuit 10' by the use of the output 9.
As may be understood from this example, very high machine accuracies are required for the prior art system. The maintenance of the required precision and proper adjustment of the equipment is therefore extremely difficult to accomplish under general surrounding conditions, as found in homes, for example. Another disadvantage is the high cost of the system.